1. Field of the Invention
The present invention relates to an Al--Ni--Y alloy thin film for use as electrode of semiconductor devices and to an Al--Ni--Y alloy target used to produce such an Al--Ni--Y alloy thin film. More particularly, the present invention relates to an Al--Ni--Y alloy film particularly suitable for forming electrodes (electrodes themselves and thin film interconnections) in an active matrix liquid crystal display and to a sputtering target used to produce such Al--Ni--Y alloy films.
2. Description of the Related Art
Liquid crystal displays (hereinafter referred to simply as LCDS) have the advantage of being smaller in thickness, being lighter in weight, and consuming less electric power than a conventional CRT display, and still being capable of displaying a high-resolution image. Liquid crystal displays are now widely used in many applications such as television sets and notebook-type personal computers. To improve the image quality such as resolution and contrast, it has been proposed to use thin film transistors (hereinafter referred to simply as TFTS) as switching elements of LCDs, and TFT-LCDs are now widely used. Herein, the TFT refers to an active device comprising a semiconductor film formed on an insulating substrate such as a glass substrate and also comprising electrodes (electrodes and interconnections in the form of a thin film) made of metal films wherein the electrodes are connected to the semiconductor film. That is, the term "electrode" of a semiconductor device is used to describe an electrode which is a part of a TFT wherein the "electrode" may be an electrode itself or an interconnection in the form of a thin film (the term "electrode" is used in a similar manner elsewhere in this description). In a TFT in a complete form finally obtained after the production process, interconnections are electrically connected to corresponding electrodes.
The film used to form the electrode of a semiconductor device in the LCD (hereinafter referred to simply as a electrode film) is required to have various characteristics. Of these various characteristics, low resistivity and high hillock resistance are particularly important. The importance of low resistivity is first described below. The resistivity of a material used to form the electrode film in the LCD has an influence on the signal pulse delay. Especially, if a material having a high resistivity is used to form the thin electrode, then the high resistivity causes a reduction in the conduction speed of the electric signal and thus an increase in the signal pulse delay. Such a signal pulse delay is an important factor which determines the overall characteristics of the LCD. In particular, in large-sized and high-resolution LCDs which are becoming popular recently, the low-resistivity is the most important factor to prevent the delay in the signal.
The hillock resistance is described below. The material used to form the electrode film is repeatedly subjected to annealing at 300.degree. C. to 400.degree. C. after the formation of the electrode film during the process of producing the film transistor. More specifically, in the LCD production, repeated annealings are required for example in the process of forming a silicon semiconductor layer after the formation of the electrode film. When aluminum is employed as the material to form the electrode film in the LCD, the problem is that small-sized hemispherical protrusions called hillocks are produced on the surface during plural heat treatments. The hillocks in the form of hemispherical protrusions are produced due to compressive stress caused by the difference in the thermal expansion coefficient between the glass substrate and the aluminum film. In TFT-LCDS, the electrode film is generally located at the bottom of the multilayer structure. Therefore, the hillocks on the electrode film make it impossible to form other films thereon in a flat shape. Furthermore, when an insulating film is formed on the electrode film, if hillocks are produced on the electrode film, the hillocks can protrude the insulating film and thus causing an electric short circuit (electric insulation failure) between layers.
Thus, in addition to the property of low resistivity, the material for the electrode film in the LCD is required to have the property that no hillocks are produced during annealings. To meet the above requirement, a refractory metal such as Ta, Mo, Cr, or Ti is used as the electrode material of an LCD on which TFTs are mounted (hereinafter such a type of LCD is referred to as a TFT-LCD). However, although these materials are excellent in the property of producing less hillocks (herein such a property is referred to as "hillock resistance"), they have a high electric resistivity when they are in the form of a film. More specifically, the resistivities of pure Ta, Mo, Cr, and Ti film are about 180, 50, 50, and 80 .mu..OMEGA.cm, respectively. These values are much higher than the required value or 5 .mu..OMEGA.cm.
The electrode materials for semiconductor devices having a low resistivity include Au, Cu, and Al. However, Au is difficult to etch into a desired pattern after depositing it into the form of a film or electrode film (interconnection film). The other disadvantage of Au is its high cost. Cu is poor in its adhesion to a substrate and also poor in corrosion resistance. Thus Au and Cu are not suitable in practical use. On the other hand, Al is poor in the hillock resistance.
In view of the above, the inventors of the present invention have proposed an electrode material for semiconductor devices which is excellent in both the low resistivity and the hillock resistance. That is, a film made of an Al alloy such as Al--Ta and Al--Ti has been disclosed in Japanese Unexamined Patent Publication No. Hei 5-100248, and a film made of an Al--Fe alloy or an Al alloy containing an element of similar to Fe (the element may be one of or two or more elements selected from the group consisting of Fe, Co, Ni, Nd, Ru, Rh, and Ir, hereinafter such alloys are collectively referred to as Al--Fe or -similar element alloy) has been disclosed in Japanese Unexamined Patent Publication No. Hei7-45555.
In recent technology of LCDs, there is an increasing tendency toward a larger size of the panel and a higher resolution. As a result of such a tendency, it has become required to form the electrode film of the LCD into smaller dimensions. Such a change in the shape toward smaller dimensions results in an increase in the electric resistance of interconnections and electrodes thus resulting in an increase in the delay time of the electric signals, which brings about a great problem in realizing high-performance LCDs. To suppress the signal pulse delay, caused by the reduction in the size of the film electrode in the LCD, to a sufficiently low level to be applicable to color LCDs in particular to large-sized color LCDs having a panel size of for example 10 inches or greater, it is required that the electric resistivity of the electrodes of semiconductor devices should be equal to or less than 5 .mu..OMEGA.cm.
When a film is made of an Al alloy such as Al--Ta, Al--Ti, or Al--Nd or Al--Fe or -similar element alloy, or although the high hillock resistance is improved by the presence of Ta, Ti, Fe or similar elements, or Nd in the alloy, the presence of such an element in the alloy also causes an increase in the electric resistivity. Therefore, such an Al alloy cannot satisfy both requirements in terms of the low resistivity less than 5 .mu..OMEGA.cm and the high hillock resistance.
As described above, the electrode film is generally subjected to repeated annealings during the LCD production process. The material of the electrode film is required to have a high hillock resistance during the repeated annealings. However, although the thin film made of an alloy such as Al--Ta, Al--Ti, Al--Fe or -similar element alloy, or Al--Nd has a high hillock resistance during the first annealing, hillocks are produced during the second or the following annealings. That is, the film made of this type of alloy is not sufficient in the hillock resistance against repeated annealings.
An attempt has recently been made to employ, as a gate insulating film, an anodic oxide film formed by anodizing an Al alloy of the type described above. In this technique, in addition to the requirement that the Al alloy film should have a low electric resistivity less than 5 .mu..OMEGA.cm and should have a excellent hillock resistance, there is an additional requirement that the anodic oxide film formed by anodizing the Al alloy should have a high dielectric strength. However, the alloy film made of Al--Ta, Al--Ti, Al--Fe or -similar element alloy, or Al--Nd cannot meet all requirements in terms of the low resistivity less than 5 .mu..OMEGA.cm, the high hillock resistance, and the high dielectric strength of the anodic oxide film.
Especially, the gate insulating film of thin film transistors used as switching elements in LCDs is required to have a high dielectric strength between adjacent layers. In the conventional techniques, a SiN film is employed as the gate insulating film. However, when SiN is employed, it is difficult to produce a SiN film which is perfectly free of pin holes. The dielectric strength of the SiN film is reduced by these pin holes or defects caused by particles generated during the process of depositing the SiN film. Therefore, it is difficult to obtain a sufficiently high dielectric strength using only the SiN film. To solve the above problem, the recent gate insulating film is generally formed into a two-layer structure consisting of an anodic oxide film and a SiN film deposited thereon.
Furthermore, in order to simplify the LCD production process, there is an attempt to replace the 2-layer gate insulating film with a single-layer gate insulating film made of an anodic oxide film. In this case, it is required that the anodic oxide gate insulating film formed by anodizing the electrode film of the alloy of the above-described type should have a high dielectric strength similar to that obtained in the SiN film. Especially, it is required that the anodic oxide film formed by anodizing the electrode film made of the above-described alloy should have a dielectric breakdown voltage higher than a voltage applied during the anodization process. If the dielectric breakdown voltage of the anodic oxide film is lower than the voltage applied during the anodization process, then an electric short circuit (electric insulation failure) is produced through the gate insulating film of TFTs, which can cause the TFTs not to operate normally. However, when an aluminum alloy such as Al--Ta, Al--Ti, Al--Nd, or Al--Fe or -similar element alloy described above is employed as the material of the alloy film, the resultant anodic oxide film does not have a sufficient dielectric strength similar to that obtained in the SiN film, and thus it is impossible to produce the gate insulating film into the single-layer structure made of the anodic oxide film.
As described above, the conventionally used thin film materials cannot satisfy all requirements needed as the electrode film of an LCD. That is, any known thin film material used to form the electrode film of the LCD cannot satisfy all requirements in terms of the low resistivity, the high hillock resistance, and the high dielectric strength of the anodic oxide film